Conventional 3D-stereoscopic photography typically employs twin cameras having parallel optical axes and a fixed distance between their aligned lenses. These twin cameras generally produce a pair of images which images can be displayed by any of the known in the art techniques for stereoscopic displaying and viewing. These techniques are based, in general, on the principle that the image taken by a right lens is displayed to the right eye of a viewer and the image taken by the left lens is displayed to the left eye of the viewer.
For example, U.S. Pat. No. 6,906,687, assigned to Texas Instruments Incorporated, entitled “Digital formatter for 3-dimensional display applications” discloses a 3D digital projection display that uses a quadruple memory buffer to store and read processed video data for both right-eye and left-eye display. With this formatter video data is processed at a 48-frame/sec rate and readout twice (repeated) to provide a flash rate of 96 (up to 120) frames/sec, which is above the display flicker threshold. The data is then synchronized with a headset or goggles with the right-eye and left-eye frames being precisely out-of-phase to produce a perceived 3-D image.
Stereoscopic motion pictures can be produced through a variety of different methods. Though anaglyph was sometimes used prior to 1948, during the early “Golden Era” of 3-D cinematography of the 1950s the polarization system was used for most feature length movies in the United States. In the 21st century, polarization 3-D systems have continued to dominate the scene, although during the 60s and 70s some classic films were converted to anaglyph for theaters not equipped for polarization, and were even shown in 3-D on TV. In the years following the mid 80s, some movies were made with short segments in anaglyph 3D. The following are some of the technical details and methodologies employed in some of the more notable 3-D movie systems that have been developed.
Anaglyph
Anaglyph images were the earliest method of presenting theatrical 3-D and the one most commonly associated with stereoscopy by the public at large, mostly because of non-theatrical 3D media such as comic books and 3D TV broadcasts, where polarization is not practical. They were made popular because of the ease of their production and exhibition. The first anaglyph movie was invented in 1915. Though the earliest theatrical presentations were done with this system, most 3D movies from the 50s and 80s were originally shown polarized.
In an anaglyph, the two images are superimposed in an additive light setting through two filters, one red and one cyan. In a subtractive light setting, the two images are printed in the same complementary colors on white paper. Glasses with colored filters in each eye separate the appropriate images by canceling the filter color out and rendering the complementary color black.
Anaglyph images are much easier to view than either parallel sighting or cross-eyed stereograms, although the latter types offer bright and accurate color rendering, particularly in the red component, which is muted, or desaturated with even the best color anaglyphs. A compensating technique, commonly known as Anachrome, uses a slightly more transparent cyan filter in the patented glasses associated with the technique. Process reconfigures the typical anaglyph image to have less parallax.
An alternative to the usual red and cyan filter system of anaglyph is ColorCode 3-D; a patented anaglyph system which was invented in order to present an anaglyph image in conjunction with the NTSC television standard, in which the red channel is often compromised. ColorCode uses the complementary colors of yellow and dark blue on-screen, and the colors of the glasses' lenses are amber and dark blue.
The anaglyph 3-D system was the earliest system used in theatrical presentations and requires less specialized hardware.
Anaglyph is also used in printed materials and in 3D TV broadcasts where polarization is not practical. 3D polarized TVs and other displays only became available from several manufacturers in 2008; these generate polarization on the receiving end.
Polarization Systems
The polarization 3-D system has been the standard for theatrical presentations since it was used for Bwana Devil in 1952, though early Imax presentations were done using the eclipse system and in the 60s and 70s classic 3D movies were sometimes converted to anaglyph for special presentations. The polarization system has better color fidelity and less ghosting than the anaglyph system.
To present a stereoscopic motion picture, two images are projected superimposed onto the same screen through different polarizing filters. The viewer wears low-cost eyeglasses which also contain a pair of polarizing filters oriented differently (clockwise/counterclockwise with circular polarization or at 90 degree angles, usually 45 and 135 degrees, with linear polarization). As each filter passes only that light which is similarly polarized and blocks the light polarized differently, each eye sees a different image. This is used to produce a three-dimensional effect by projecting the same scene into both eyes, but depicted from slightly different perspectives. Since no head tracking is involved, the entire audience can view the stereoscopic images at the same time. Additionally, since both lenses have the same color, people with one dominant eye (amblyopia), where one eye is used more, are able to see the 3D effect, previously negated by the separation of the two colors.
In the case of RealD a circularly polarizing liquid crystal filter which can switch polarity 144 times per second is placed in front of the projector lens. Only one projector is needed, as the left and right eye images are displayed alternately. Sony features a new system called RealD XLS, which shows both circular polarized images simultaneously: a single 4K projector (4096×2160 resolution) displays both 2K images (2048×858 resolution) on top of each other at the same time, a special lens attachment polarizes and projects the images.
Thomson Technicolor has produced a system using a split lens which allows traditional 35 mm projectors to be adapted to project in 3D using over/under 35 mm film. This is a very cost-effective way to convert a screen as all that is needed is the lens and metallic (silver) screen rather than converting entirely to digital. A metallic screen is necessary for these systems as reflection from non-metallic surfaces destroy the polarization of the light.
Polarized stereoscopic pictures have been around since 1936, when Edwin H. Land first applied it to motion pictures. The so called “3-D movie craze” in the years 1952 through 1955 was almost entirely offered in theaters using linear polarizing projection and glasses. Only a minute amount of the total 3D films shown in the period used the anaglyph color filter method. Linear polarization was likewise used with consumer level stereo projectors. Polarization was also used during the 3D revival of the 80s.
In the 2000s, computer animation, competition from DVDs and other media, digital projection, and the use of sophisticated IMAX 70 mm film projectors, have created an opportunity for a new wave of polarized 3D films.
Eclipse Method
With the eclipse method, a shutter blocks light from each appropriate eye when the converse eye's image is projected on the screen. The projector alternates between left and right images, and opens and closes the shutters in the glasses or viewer in synchronization with the images on the screen. This was the basis of the Teleview system which was used briefly in 1922.
A variation on the eclipse method is used in LCD shutter glasses. Glasses containing liquid crystal that will let light through in synchronization with the images on the cinema, TV or computer screen, using the concept of alternate-frame sequencing. This is the method used by nVidia, XpanD 3D, and earlier IMAX systems. A drawback of this method is the need for each person viewing to wear expensive, electronic glasses that must be synchronized with the display system using a wireless signal or attached wire. The shutterglasses are heavier than most polarized glasses though lighter models are no heavier than some sunglasses or deluxe polarized glasses. These systems, however, do not require a silver screen for projected images.
Interference Filter Technology
Dolby 3D uses specific wavelengths of red, green, and blue for the right eye, and different wavelengths of red, green, and blue for the left eye. Eyeglasses which filter out the very specific wavelengths allow the wearer to see a 3D image. This technology eliminates the expensive silver screens required for polarized systems such as RealD, which is the most common 3D display system in theaters. It does, however, require much more expensive glasses than the polarized systems. It is also known as spectral comb filtering or wavelength multiplex visualization
The recently introduced Panavision 3D system also uses this technology, though with a wider spectrum and more “teeth” to the “comb” (5 for each eye in the Panavision system). Panavision also claim that their glasses are cheaper to manufacture than those used by Dolby.
Pulfrich
The Pulfrich effect is based on the phenomenon of the human eye processing images more slowly when there is less light, as when looking through a dark lens.
Imagine a camera which starts at position X and moves right to position Y. If a viewer watches this segment with a dark lens over the left eye, then when the right eye sees the image recorded when the camera is at Y, the left eye will be a few milliseconds behind and will still be seeing the image recorded at X, thus creating the necessary parallax to generate right and left eye views and 3D perception, much the same as when still pictures are generated by shifting a single camera. The intensity of this effect will depend on how fast the camera is moving relative to the distance to the objects; greater speed creates greater parallax. A similar effect can be achieved by using a stationary camera and continuously rotating an otherwise stationary object. If the movement stops, the eye looking through the dark lens (which could be either eye depending on the direction the camera is moving) will “catch up” and the effect will disappear. One advantage of this system is that people not wearing the glasses will see a perfectly normal picture.
Of course, incidental movement of objects will create spurious artifacts, and these incidental effects will be seen as artificial depth not related to actual depth in the scene. Unfortunately, many of the applications of Pulfrich involve deliberately causing just this sort of effect and this has given the technique a bad reputation. When the only movement is lateral movement of the camera then the effect is as real as any other form of stereoscopy, but this seldom happens except in highly contrived situations.
Though Pulfrich has been used often on TV and in computer games, it is rarely if ever used in theatrical presentations.
Spectral Separation
ChromaDepth uses a holographic film in the glasses that creates an effect like a dispersive prism. This causes redder objects to be perceived as near and bluer objects as farther away.
Lenticular or Barrier Screens
Lenticular printing and parallax barrier technologies involve imposing two (or more) images on the same sheet, in narrow, alternating strips, and using a screen that either blocks one of the two images' strips (in the case of parallax barriers) or uses equally narrow lenses to bend the strips of image and make it appear to fill the entire image (in the case of lenticular prints). To produce the stereoscopic effect, the person must be positioned so that one eye sees one of the two images and the other sees the other. In this method, glasses are not necessary to see the stereoscopic image.
Both images are projected onto a high-gain, corrugated screen which reflects light at acute angles. In order to see the stereoscopic image, the viewer must sit within a very narrow angle that is nearly perpendicular to the screen, limiting the size of the audience. Lenticular was used for theatrical presentation of numerous shorts in Russia from 1940-1948 and in 1954 for the feature length films Crystal, Machine 22-12 and The Pencil on Ice.
Though its use in theatrical presentations has been rather limited, lenticular has been widely used for a variety of novelty items and has even been used in amateur 3D photography.
All the above described methods share a common drawback—they all require stereoscopic photography. This means that only images originally photographed/filmed with specialized equipment may be viewed in 3D. It would therefore be desirable to create stereoscopic images/video from conventional images/videos photographed/filmed using conventional (e.g. single aperture) photographic equipment.